In the semiconductor industry, chemical mechanical polishing (CMP) is used to selectively remove portions of a film from a semiconductor wafer by rotating the wafer against a polishing pad (or rotating the pad against the wafer, or both) with a controlled amount of pressure in the presence of a chemically reactive slurry. Overpolishing (removing too much) or underpolishing (removing too little) of a film results in scrapping or rework of the wafer, which can be very expensive. Various methods have been employed to detect when the desired endpoint for removal has been reached, and the polishing should be stopped. One such method described in U.S. Pat. No. 5,559,428 entitled "In-Situ Monitoring of the Change in Thickness of Films," assigned to the present assignee, uses a sensor which can be located near the back of the wafer during the polishing process. As the polishing process proceeds, the sensor generates a signal corresponding to the film thickness, and can be used to indicate when polishing should be stopped.
Generating the signal and using the signal to control the CMP process for automatic endpoint detection are two different challenges, however. During polishing, different conditions may arise which can result in the signal falsely indicating that the endpoint has been reached. For example, the film can be locally non-planar (i.e. "cupped") under the sensor, or the film can be multi-layered (i.e. one type of metal over another). In each of these cases, the change in thickness of the film may not be constant and can even stop for a while under the sensor, so that a false endpoint can be detected. Another issue arises due to the fact that while a single sensor can respond to the thickness of a film in the immediate vicinity, it cannot directly monitor the entire film area on the wafer. Thus a certain amount of overpolishing is necessary to ensure that the entire film has been polished, and a way to determine the correct amount of overpolishing. In addition, the polishing process should be able to be easily and quickly custom-tailored to polishing different types of films, so that down time between lots is minimized. Finally, operator training should be easy, with minimal scrapping of wafers, and a polishing history for each wafer kept so that problem determination and resolution is simplified.
These challenges were met with a chemical mechanical polishing endpoint process control system described in U.S. Pat. No. 5,659,492, which is incorporated herein in its entirety. This process control system functions well for the type of polishing setup and monitoring described above. However, when used with alternate methods of CMP monitoring, especially CMP processes that (1) have a signal trace with different characteristics (i.e. different flat regions and sloped regions), (2) reach endpoint very quickly, with a small operating window for accuracy, and (3) involve a monitoring setup that reflects polishing across the entire wafer rather than sensing a specific location, the control system lacks accuracy and robustness.
Thus there remains a need for a more accurate and robust system for detecting and determining the endpoint for chemical-mechanical polishing. Such a system should capture reference points (i.e. key points in the signal trace) very quickly as well as be extremely accurate when calculating the overpolish time. It should also be suitable for use in large-scale production including preventing propagation of errors from one wafer to the next.